1. Field of the Invention
The present invention generally relates to a reflective light control (switchable mirror) element having a switchable layer, a switchable mirror component having the switchable mirror element, and an insulating glass having the switchable mirror element.
2. Description of the Related Art
Windows of a building provide large areas through which heat can enter or exit the building. A study suggests that about half of the heating energy that is lost from a typical house in winter exits via windows of the house, and as much as 70% of the heat that enters the house that is air-conditioned during summer comes through the windows. Thus, very large energy savings can be expected by properly controlling the passage of light and heat through the windows.
A switchable glass has been developed for this purpose, i.e., to control the entry and exit of light and heat. The material for the switchable glass may be categorized into several types depending on the way in which the glass controls light. They are 1) electrochromic material, in which optical transmittance can be changed reversibly by the application of a current or voltage; 2) thermochromic material, in which optical transmittance can be changed depending on temperature; and 3) gasochromic material, in which optical transmittance can be changed by controlling an atmosphere gas. Most advanced among those in terms of research and development is the electrochromic material. An electrochromic switchable glass with a switchable layer including a thin film of tungsten oxide has already reached practical application stage, and some commercial products are available.
In the electrochromic switchable glass in which a tungsten oxide thin-film is used, the switchable layer controls light by absorbing light. As a result, the switchable layer is heated as it absorbs light, and the heat is radiated back into the room or space fitted with the glass, thereby reducing the total energy saving effect. To overcome this problem, the entry or exit of light should desirably be controlled by reflecting light, rather than by absorbing it. Thus, a need had long been felt for material having properties such that it can reversibly switch between a transparent state and a mirror state.
Such material had not been found for a long time until 1996 when a Dutch group discovered that a transparent state and a mirror state can be reversibly switched by hydrogenating and dehydrogenating a rare-earth metal such as yttrium or lanthanum. A mirror using such material was named “switchable mirror” (see e.g., J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijngaarden, J. P. Dekker, D. G. de Groot, and N. J. Koeman, “Yttrium and lanthanum hydride films with switchable optical properties”, Nature, Vol. 380, 231 (1996)). Such hydrogenation and dehydrogenation of rare-earth metals can produce large changes in optical transmittance, and provide excellent switchable mirror properties. However, because such a switchable mirror requires rare-earth metals, problems concerning resources and cost have been encountered when used for coating a window or the like.
Materials known to exhibit the reflective light control properties (hereafter referred to as “switchable mirror properties”) include rare-earth metals such as yttrium and lanthanum, alloys of a rare-earth metal such as gadolinium and magnesium, and alloys of magnesium and a transition metal (see, e.g., U.S. Pat. No. 6,647,166). Among those materials, an alloy of magnesium and a transition metal is suitable for coating a window glass from the viewpoint of resources and cost. Particularly, a magnesium-nickel alloy has been reported to provide high optical transmittance in the transparent state (see, e.g., JP Patent No. 3968432). However, these known materials are not completely colorless and transparent in the transparent state, but have a yellow to brown tint in varying degrees, posing a potential problem in application for window glasses. In order to overcome this problem of tint, a switchable mirror element using a magnesium-titanium alloy has been developed (see, e.g., Japanese Laid-Open Patent Application No. 2008-152070).
However, the conventional switchable mirror element using a magnesium-titanium alloy has a lower optical transmittance in the transparent state than a switchable mirror element that uses a magnesium-nickel alloy.